Plasma processing is used in the surface treatment of a variety of materials. For example, plasmas are used in semiconductor processing to clean or etch surfaces, and in depositions on the surfaces. Plasmas may also be used in cleaning a variety of materials, including metals, and in deposition of ions, free radicals, and other species from the plasma, either onto the surface of the material to provide surface coating, or into the bulk of the material. The use of plasmas in the treating of polymer surfaces has drawn increasing attention. See, e.g., G. Menges, et al. "Plasma Polymerization--Tailored Coats for Plastic Mouldings" Kunststoffe German Plastics, Volume 78, Number 10, 1988, pp. 91-92; P. Plein, et al. "Plasmapolymerization as Coating Process for Plastic and Metallic Parts," Antec '88, 1988, pp 1338-1341; R. Ludwig, "Plasmapolymerization--A New Technology for Surface Modification," Antec '89, 1989, pp. 915-917; J. T. Felts, et al., "Commercial Scale Application of Plasma Processing for Polymeric Substrates: From Laboratory to Production," presented at 38th Annual Symposium and Topical Conferences of the American Vacuum Society, Seattle, Wash., Nov. 11-15, 1991 and published in J. Vac. Sci. and Technol., A10, p. 1675, 1992.
The ability to use a plasma to produce a surface treatment or deposition solely on the interior surface of a hollow form having a re-entrant shape such as a bottle or gas tank is of great commercial importance. For example, gases and solvents permeate into walls of various plastics such as polyethylene terephthalate (PET) and high density polethylene (HDPE), that are commonly used for producing containers used in packaging. Permeation of materials reduces the shelf life of products in such packaging and can leave the container unsuitable for future packaging applications. By using plasma deposition to deposit a barrier coating film such as hexamethyldisiloxane (HMDSO) on the interior surface of the container, such permeation through the container may be severely curtailed, and shelf life greatly extended. Also, plasma treatments may be used to treat the interior surfaces of glass and plastic containers to alter the appearance of the surface or to enhance adhesion of metals or plastic materials to the treated container.
In using plasma processing to treat containers, it is important to ensure that the plasma discharge breaks down solely inside the container to be treated rather than inside the overall vacuum chamber where the plasma processing takes place. The reaction should occur only inside the container, but not inside the vacuum chamber. This would be beneficial, cutting down on high maintenance requirements of vacuum chambers within which deposition is occurring.
One of the largest maintenance problems occurring in plasma deposition apparatuses is thick film formation on the plasma source components and interior surfaces of the vacuum chamber in which the deposition takes place. After plasma-deposited films reach a thickness greater than 10 .mu.m, internal stresses in the film may delaminate it. Massive delamination results in undesirable macroscopic particles which can contaminate further processing. These macroscopic particles are generated by films deposited on source parts or on vacuum chamber walls. Time consuming maintenance must be performed to remove these films and particulates. Thus, it is desirable to have a plasma processing source in which preferably no film is ever deposited on the power coupling components or interior walls of the vacuum chamber.
Most conventional plasma generating machines are not designed to treat polymeric bodies and forms. However, a few methods are known by which plasma sources have been adapted to deposit films on the interior of a hollow form. One method which has been adapted to deposit films on the interior of a hollow form utilizes a microwave driven plasma source. See, e.g., P. Plien, et al., "Plasmapolymerization as Coating Process for Plastic and Metallic Parts," supra. This approach has some technical complications, however. Microwave power sources and wave-guide components are expensive and often difficult to custom design. Also, to avoid non-uniformities in processing complex forms, electromagnetic field scatterers or workpiece rotation may be required to disrupt "hotspots", as in conventional over-molded microwave cavity ovens.
Another previous system designed for plasma-assisted film deposition or treatment of hollow containers consists of a capacitively coupled plasma system to drive a low pressure gas discharge within the form. See German Patent No. DE 3,908,418, H. Grunwald, issued Sep. 20, 1990. Such a method also has disadvantages, including a potentially lower deposition and treatment rate for mass produced applications. In addition, capacitively coupled plasma systems utilize high plasma sheath energies which may result in excess heating of sensitive plastic walls of the container resulting in container damage. This design is also complicated and may require expensive and regular maintenance due to film deposition on power coupling components.
Present inductively-coupled plasma processing systems also have disadvantages. Such systems require expensive conformal dielectric vacuum boundaries, and may be difficult to adapt to a wide variety of forms with complex geometries.